1. Field of the Invention
This invention relates to a method of forming bottomed cylindrical bodies from a cylindrical body of thermoplastic resin. The bottomed cylindrical body can be directly used as a container, but can be conveniently used as a preform for producing a final desired container by a molding technique known per se, such as stretch blow-molding.
2. Description of the Prior Art
In recent years, mono-or multilayer containers of thermoplastic resin have gained widespread acceptance in place of glass or metallic containers for holding various articles such as foods and drinks and detergent liquids. Such containers are generally produced by forming a preform in the form of a bottomed cylindrical body from a thermoplastic resin stock, and then stretch blow-molding the preform.
The preform is most generally produced by an injection molding method. There has also been proposed and gained commercial acceptance a molding method, called an extrusion pipe method, which comprises forming a pipe with both ends open, i.e. a cylindrical body, by extrusion molding, thereafter cutting the pipe to a predetermined length, and deforming one end portion of the pipe to form a bottomed cylindrical body. This method is described, for example, in "Molding Apparatus and Techniques for PET Bottles" at pages 86-94 of "Plastics Age", October 1982 (published by Plastics Age Co., Ltd.). As described in this publication, the extrusion pipe method has various advantages over the injection molding method. But the conventional extrusion pipe method has the following problems or defects which have to be overcome.
According to the conventional extrusion pipe method, when the cylindrical body having both open ends is converted to a bottomed cylindrical body by deforming its one end portion, the one end portion of the cylindrical body is heat-melted, and simultaneously with, or subsequently to, the melting, the one end portion in the molten state is directly deformed into the desired bottom shape by an inside press mold and an outside press mold. Experiences of the present inventors, however, tell that when this method is used to form the bottomed cylindrical body, so-called neck wrinkles occur at the bottom portion deformed to the required shape, and lead to a lack of thickness uniformity and aesthetic beauty in a final container molded from the bottomed cylindrical body, and that when the cylindrical body is of a laminated structure composed of multiple thermoplastic resin layers, the laminated structure is considerably disordered in the aforsaid bottom portion and this portion becomes defective both in strength and in performance.